remove attribute name/type from instrument creation (#1594)

Author: reyang

specification/metrics/new_api.md

@@ -153,8 +153,6 @@ will have the following information:
   instruments, whether it is synchronous or asynchronous
 * An optional `unit of measure`
 * An optional `description`
-* An optional list of [`Attribute`](../common/common.md#attributes) names and
-  types
 
 Instruments are associated with the Meter during creation, and are identified by
 the name:
@@ -243,8 +241,6 @@ The API MUST accept the following parameters:
   rule](#instrument-unit).
 * An optional `description`, following the [instrument description
   rule](#instrument-description).
-* An optional list of [`Attribute`](../common/common.md#attributes) names and
-  types.
 
 Here are some examples that individual language client might consider:
 
@@ -319,6 +315,14 @@ for the interaction between the API and SDK.
 * A value
 * [`Attributes`](../common/common.md#attributes)
 
+## Compatibility
+
+All the metrics components SHOULD allow new APIs to be added to existing
+components without introducing breaking changes.
+
+All the metrics APIs SHOULD allow optional parameter(s) to be added to existing
+APIs without introducing breaking changes.
+
 ## Concurrency
 
 For languages which support concurrent execution the Metrics APIs provide